Synthesis of bisnitropyrazolylmethane and dialkyl-dinitro-imidazolium as candidates for new high energy materials

Abstract

아졸계 화합물에 다수의 니트로기가 치환될 경우, 폭발성능은 향상되지만, 증가된 전자끌기 유발효과로 인해 친핵성이 저하되어 알킬화 반응으로 새로운 구조의 고에너지 물질을 합성하는 시도들이 어려움을 겪어왔다. 본 논문에서는 반응조건의 조절과 높은 친전자성을 가진 알킬화 반응 시약의 사용을 통해 기존 합성이 어려운 것으로 보고된 새로운 고에너지 물질들을 합성한 결과를 제시한다. 첫째로, 2개의 강한 활성감소 치환기인 니트로기와 1개의 약한 활성감소 치환기인 염소기가 치환되어 기존 메틸렌 결합으로 커플링 반응이 어려운 것으로 보고된 4-chloro-3,5-dinitro-1H-pyrazole (ClDNP) 1의 커플링 반응을 통해 새로운 고에너지 물질 합성을 시도하였다. 친전자성이 높은 디할로메텐인 디아이오도메테인을 과량으로 사용한 반응조건에서 ClDNP로부터 bis(4-chloro-3,5-dinitro-1H-pyrazol-1-yl)methane 2을 79%의 수율로 얻을 수 있었다. 이러한 과량 반응조건은 친핵성이 저하된 니트로아졸계 물질의 커플링 반응에 적용할 수 있을 것으로 기대된다. 또한, 아지도화 반응을 통해 염소기를 고에너지기인 아지도기로 치환하여 새로운 1차 폭발물 후보로서 bis(4-azido-3,5-dinitro-1H-pyrazol-1-yl)methane 3을 77% 수율로 얻을 수 있었다. 실측 및 계산을 통해 분석된 3의 물성은 군사적으로 널리 사용되는 1차 폭발물인 DDNP (2-diazo-4,6-dinitrophenol)보다 우수한 열적 안정성과 폭발성능을 가진 것으로 확인되었다. 둘째로, 2개의 강한 활성화 치환기인 니트로기 치환으로 N,N-디알킬화 반응이 어려운 것으로 보고된 4,5-dinitro-1H-imidazole 4와 1-methyl-4,5-dinitro-1H-imidazole 5를 quaternization하여 새로운 에너지 이온성 액체(energetic ionic liquids, EILs)를 합성하였다. 강한 친핵성을 가진 것으로 보고된 Meerwein 시약을 사용하여 무수조건에서 반응하여 새로운 EILs인 1,3-diethyl-4,5-dinitro-1H-imidazol-3-ium tetrafluoroborate 6과 3-ethyl-1-methyl-4,5-dinitro-1H-imidazol-3-ium tetrafluoroborate 7를 각각 얻을 수 있었다. 이 반응조건은 활성감소 치환기로 인해 친핵성이 저하된 헤테로고리 화합물의 quaternization에 적용 가능할 것으로 기대된다. 또한, 폭발성능을 가진 potassium 2,4,5-trinitroimidazol-1-ide로 음이온 metathesis하여 새로운 고에너지 물질인 3-diethyl-4,5-dinitro-1H-imidazol-3-ium 2,4,5-trinitroimidazol-1-ide 8와 3-ethyl-1-methyl-4,5-dinitro-1H-imidazol-3-ium 2,4,5-trinitroimidazol-1-ide 9을 각각 얻을 수 있었다. 이 중에서, 화합물 9은 대표적인 고에너지 물질인 TNT에 비해 우수한 둔감성과 폭발성능을 동시에 갖춘 것으로 확인되었다.

The azole compounds substituted with nitro groups have a difficulty in that their explosive performance is improved but their nucleophilicity is decreased as electron-withdrawing nitro groups are additionally introduced. For this reason, it remains a challenging task to synthesize new high-energy materials (HEMs) via alkylation of the nitroazoles. In this thesis, the results of synthetic studies for new nitroazoles based HEMs are presented via their alkylation reactions using selected reagents with high electrophilicity or applying appropriate reaction conditions. First, the N,N'-methylene bridged coupling reaction of 4-chloro-3,5-dinitro-1H-pyrazole (ClDNP) 1 was studied. ClDNP 1 is known to have a difficulty for coupling reaction due to its reduced nucleophilicity. Bis(4-chloro-3,5-dinitro-1H-pyrazol-1-yl)methane 2 was obtained in a yield of 79% under reaction conditions in which highly electrophilic diiodomethane was used excessively. It is expected that these reaction conditions of using the excess alkylation reagent could be applied to the coupling reaction of the nitroazole-based material having reduced nucleophilicity. Compound 2 is expected to be a useful precursor of new HEMs because it is easy to change its reactive chloro groups with energetic functional groups. In fact, bis(4-azido-3,5-dinitro-1H-pyrazol-1-yl)methane 3 was obtained in 77% yield by substituting the chloro group of 2 with energetic azido group under mild reaction condition. Compound 3 is expected to be used as a new green primary explosive candidate as it has better thermal stability and explosive performance than DDNP (2-diazo-4,6-dinitrophenol), a widely used military primary explosive. Second, the synthesis of 4,5-dinitro-N,N'-dialkylimidazolium cations was studied by quaternizing 4,5-dinitroimidazoles via alkylation reactions. In the research field of energetic ionic liquids (EILs), dinitroimidazoles have been reported to be unable to be quaternized due to the electron-withdrawing effect nitro groups substituted on them. In order to achieve this challenging task, it was necessary to select a more reactive alkylating reagent than methyl triflate. This challenge was overcome by using Meerwiens reagents (R3OBF4) as more reactive alkylating reagents. As a result of quaternization reaction of 4,5-dinitro-1H-imidazole 4 and 1-methyl-4,5-dinitro-1H-imidazole 5 using triethyloxonium tetrafluoroborate (Et3O+BF4-), 1,3-diethyl-4,5-dinitro-1H-imidazol-3-ium tetrafluoroborate ([1,3-diEt-4,5-diNO2-Im][BF4]) 6 and 3-ethyl-1-methyl-4,5-dinitro-1H-imidazol-3-ium tetrafluoroborate ([3-Et-1-Me-4,5-diNO2-Im][BF4]) 7 were obtained. Newly obtained compounds 6 and 7 are expected to be used as precursors for new HEMs via metathesis reaction with energetic anions. In fact, two new HEMs candidates, 1,3-diethyl-4,5-dinitro-1H-imidazol-3-ium 2,4,5-trinitroimidazol-1-ide ([1,3-diEt-4,5-diNO2-Im][2,4,5-triNO2-Im]) 8, 3-ethyl-1-methyl-4,5-dinitro-1H-imidazol-3-ium 2,4,5-trinitroimidazol-1-ide ([3-Et-1-Me-4,5-diNO2-Im][2,4,5-triNO2-Im]) 9 were also obtained via anion metathesis reaction with energetic 2,4,5-trinitroimidazolate anion. Compound 9 showed the promising properties as a new insensitive HEM because of its excellent insensitivity and more powerful explosive performance than those of TNT (2,4,6-trinitrotoluene).